of the invisible Rays oj the Sun. 291 



inch and a half beyond the red ones, measured upon their pro- 

 jection on a horizontal plane. I have no doubt but that their 

 efficacy may be traced still somewhat farther. 



The 5th and 6th experiments shew, that the power of heating 

 is extended to the utmost limits of the visible violet rays, but 

 not beyond them ; and that it is gradually impaired, as the rays 

 grow more refrangible. 



The four last experiments prove, that the maximum of the 

 heating power is vested among the invisible rays ; and is pro- 

 bably not less than half an inch beyond the last visible ones, 

 when projected in the manner before mentioned. The same 

 experiments also shew, that the sun's invisible rays, in their less 

 refrangible state, and considerably beyond the maximum, still 

 exert a heating power fully equal to that of red-coloured light ; 

 and that, consequently, if we may infer the quantity of the effi- 

 cient from the effect produced, the invisible rays of the sun 

 probably far exceed the visible ones in number. 



To conclude, if we call light, those rays which illuminate 

 objects, and radiant heat, those which heat bodies, it may be 

 inquired, whether light be essentially different from radiant 

 heat? In answer to which I would suggest, that we are not 

 allowed, by the rules of philosophizing, to admit two different 

 causes to explain certain effects, if they may be accounted for 

 by one. A beam of radiant heat, emanating from the sun, 

 consists of rays that are differently refrangible. The range of 

 their extent, when dispersed by a prism, begins at violet- 

 coloured light, where they are most refracted, and have the 

 least efficacy. We have traced these calorific rays throughout 

 the whole extent of the prismatic spectrum ; and found their 

 power increasing, while their refrangibility was lessened, as far 



